Concentration measurement by sensing hydrogel pressure

ABSTRACT

A sensor and method for measuring the concentration of one or more chemical substances within a solution. The sensor including a body of saturated hydrogel held within a sensor body. A window to allow flow through of the chemical substance to be measured. The hydrogel is held under a predetermined compressive force within the sensor body such that the hydrogel exerts a base pressure. A pressure sensor such as a strain gauge senses the pressure exerted by the hydrogel within the sensor body. The sensed pressure is stored or displayed. The hydrogel is reactive to the one or more chemical substances so that contact causes the hydrogel to either increase the pressure above the base pressure under which the hydrogel is held or to decrease the pressure relative to the base pressure of the hydrogel.

This invention relates to a method and apparatus for measuring theconcentration of a chemical substance within a solution, and in onespecific aspect the dynamic measurement of that concentration

BACKGROUND OF THE INVENTION

There are several circumstances where it is desired to measure thechange in concentration of chemical substances in a solution in adynamic fashion. In the wine making industry the levels of sugar andalcohol are measured in order to determine the extent to which thefermentation process should be continued. Similarly with the beerindustry. Traditionally such industries measure the level of sugar bymeasuring the density of the solution by refractometry. Such a practicerequires opening of the fermentation vat, with attendant risk ofcontamination, removing a sample and measuring. The method entailsdetermining the refractive index of the solution which gives anindication of the density. A concordance table is consulted to determinethe quantity of sugar. A calculation is then made to determine thereduction in sugar concentration relative to the starting concentrationand from that a measure of the alcohol concentration is calculated. Thiscalculated value gives only an approximation, requires the recordal ofdata manually and a direct sampling from the vat.

There have been suggested a number of approaches to measuringconcentrations of various molecules, such as sugars in a dynamicfashion, and these include the use of an osmotic cell such as disclosedin U.S. Pat. No. 5,005,403 in the name of Steudle et al. Thisarrangement is quite complex and the document makes no disclosure ofmeasuring the cumulative effects of multiple chemicals.

Another approach to measuring the presence of glucose is to use aspecifically reactive, pH sensitive, hydrogel which has a glucosereactive linked enzyme the product of which reaction leads to changes inpressure emanating from the hydrogel. (see U.S. Pat. No. 6,268,161 toHan et al.,) This is a relatively complex arrangement suitable forspecialised uses.

Hydrogels have been used in the past for monitoring. One such disclosureis WO 00/37935 by the present inventor whereby hydrogels held under acompressive force have been used to measure the matrix potential ofsoil.

OBJECT OF THE INVENTION

The object of the invention is to provide for a sensor that in a simpleyet effective manner is able to determine the concentration of achemical substance within a solution in a simple yet effective manner,or at least to provide the public with a useful alternative.

SUMMARY OF THE INVENTION

This invention arises from an arrangement whereby saturated hydrogelheld under a predetermined pressure reacts to certain chemical moietiesby increasing or decreasing the pressure in a manner directly, orinversely, related to the concentration of the chemical moietyconcerned. This has been determined in particularly for sugar andalcohol which both are found to have effects on particular hydrogelsheld under pressure that are opposite in direction and quantitativelydifferent so that this relationship can be used, in one specific form ofthe invention, in the wine industry. Other moieties also interact withhydrogels either singly or in combination and this invention thereforehas broader applicability.

In a broad form of a first aspect this invention might be said to residein a sensor for measuring the concentration of one or more chemicalsubstances within a solution, said sensor including a body of saturatedhydrogel held within a sensor body, said sensor body having at least awindow to allow flow through of the one or more chemical substances tobe measured, the hydrogel is held under a predetermined compressiveforce within the sensor body such that the hydrogel exerts a basepressure, the sensor further includes a pressure sensing means to sensethe pressure exerted by the hydrogel within the sensor body, thepressure sensing means communicative with a storage or display means tostore or display a sensed pressure, said hydrogel reactive to the one ormore chemical substance which cause the hydrogel to either increase thepressure above the base pressure under which the hydrogel is held or todecrease the pressure relative to the base pressure of the hydrogel.

In an alternative broad form the invention could be said to reside in amethod of monitoring the concentrations of one or more chemicalsubstances in a solution said monitoring including the holding hydrogelunder a predetermined pressure, exposing the hydrogel to the solution,recording the change in pressure in the hydrogel over time, the hydrogelchosen such that the one or more chemical substances exerts an expansiveor a contractive effect on the hydrogel at the predetermined pressureapplied to the hydrogel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view from below of a first embodiment of a sensor,showing the body of the sensor and window for ingress of the solution,

FIG. 2 is a cross sectional view through II-II of FIG. 1 showing detailof the manner in which a hydrogel is held in the body of the sensor by aporous ceramic disk which is further supported by a grate which definesthe windows for ingress of the solution, the outside body of the sensorand a securing collar for securing the hydrogel and pressure transducerwithin the housing,

FIG. 3 is a plan view from above showing the layout of piezo electriccrystal overlaying the bridge of the body of the first embodiment ofsensor,

FIG. 4 is a layout of the manner in which the transducer functions andwhere the signals are processed,

FIG. 5 is a similar cross sectional view of a second embodiment of asensor according to the present invention,

FIG. 6 is a graph of the output from the sensor with varyingconcentrations of sodium chloride,

FIG. 7 is a graph of the output from the sensor with varyingconcentrations of sugar,

FIG. 8 is a graph of the output from the sensor with varyingconcentrations of alcohol.

DETAILED DESCRIPTION OF THE INVENTION

The core of the sensor is a hydrogel that is held under a predeterminedpressure. The compressive force leading to the pressure is to take thehydrogel to a state in which further changes in pressure in response tochanges in concentration of certain chemical moieties has a predictablerelationship. The pressure might be zero but preferably within a rangefrom about 50 Kpa to about 400 kPa, more preferable between 100 and 300kPa and most preferably about 200 Kpa. It is found that estimates ofconcentrations are more reliable when the hydrogel is held under apositive pressure, and in particular where exposure to any one of thechemical substances to be measured leads to a reduction in pressure byshrinkage of the hydrogel. The extent of the predetermined pressuremight be such that it is sufficient to accommodate an anticipateddecrease in pressure occasioned by measuring a range of concentrationsof chemical substances causing reducing in pressure or shrinkage of thehydrogel.

Hydrogel are polymeric materials which swell in water and othersolvents, absorbing fluid within the polymer network without dissolving.Hydrogels suitable for this invention are the expandable, drivingswellable hydrophilic polymers, known as osmopolymers. The swellablehydrophilic polymers are cross-linked and may be lightly cross-linked,such cross-links being formed by covalent or ionic bond, which interactwith water and aqueous biological fluids and swell or expand to someequilibrium state. The hydrogels can be of plant and animal origin, andmay be prepared by modifying naturally occurring structures, andsynthetic polymer hydrogels. The hydrogels exhibit considerableexpansive capacity generally ranging from a 2 to 50 fold volumeincrease. Hydrogels useful for this invention may be selected from thefollowing polymers, poly(hydroxyalkyl methacrylate),poly(N-vinyl-2-pyrrolidone), anionic and cationic hydrogels,polyelectrolyte complexes, poly(vinyl alcohol) having a low acetateresidual and cross-linked with glyoxal, formaldehyde, or glutaraldehyde,methyl cellulose cross-linked with dialdehyde, a mixture of cross-linkedagar and carboxymethyl cellulose, a water insoluble, water-swellablecopolymer produced by forming a dispersion of finely divided copolymerof maleic anhydride with styrene, ethylene, propylene, butylene, orisobutylene cross-linked with from 0.001 to about 0.5 moles of apolyunsaturated cross-linked agent per mole of maleic anhydride in thecopolymer, water-swellable polymers of N-vinyl lactams, cross-linkedpolyethylene oxides, and the like. Hydrogels are referred to in a numberof US patent documents including the following U.S. Pat. Nos. 3,865,108,4,022,173, 4,207,893 and in Handbook of Common Polymers by Scott andRoff, published by the Chemical Rubber Company, Cleveland, Ohio.

These hydrogels may be further modified to have specific or particularaffinities for the one or more chemical substances the concentration ofwhich is to be determined. Alternatively modification might be desiredto repel or minimise the interaction of other compounds which wouldadversely impact on the reliability of the change in pressure.

The hydrogel will be selected to react with the chemical compound suchthat varying concentrations will either increase or decrease thepressure exerted by the hydrogel on the pressure sensor relative to thebase pressure.

A hydroxyalkyl methacrylate hydrogel specifically a 2-hydroxyethylmethacrylate held at a base pressure of 200 kpa reacts with alcohol atlevels relevant to wine and beer manufacture in an direct relationshipwith the concentration. The same hydroxyalkyl methacylate hydrogel isfound to have an inverse relationship with sucrose, but quantitativelythe relationship is different by a factor of about 3.2. As one mightexpect therefore, over time the fermentation of wine it was found thatthe pressure increases with time. The change in pressure exerted by thehydrogel over the course of wine fermentation might thus be expected tobe a composite of the curves of the pressure changes exerted by a changein the concentration of sugar and the change in concentration of alcoholand other reactive substances in the fermentation vat. Trial conductedon wine fermentation show that using the above hydrogel that the changein pressure exerted by the hydrogel follows closely the curve in changein sugar concentration as measured by conventional techniques.

It is anticipated that in the case of dynamically measuring afermentation process leading to the production of an alcohol from asimple or complex sugar that a range of hydrogels can be used, and thesemay be determined empirically. It is anticipated that certain hydrogelswill have no or minimal reaction to the sugars fermented and will reactprimarily or solely to alcohol, and conversely other hydrogels willreact primarily with the sugar concerned and minimally or not at allwith alcohol. Candidate hydrogels can be tested for suitability bytesting against varying concentrations of sugar and alcohol separatelybefore being trialled in a fermentation.

From the above it will be understood that the hydrogel itself exerts theselectivity of reaction between two competing influences on pressure. Itwill be understood that other solutes may well also change and these arealso likely to have an influence on the pressure exerted by thehydrogel. It is found with experimentation with wine and beer makingthat whatever other changes occur these are overshadowed by the changein concentration of sugar and alcohol, and that then gives a goodmeasure of the extent to which fermentation has proceeded.

Whilst wine and beer making have been highlighted this finding will alsobe applicable to other fermentations that result in the production ofalcoholic or industrial alcohols. Or indeed where alcohol is furthermodified such as in the production of vinegar where it might be desiredto measure the decrease in the amount of alcohol in favour of vinegar.Alternative uses might be where the mixture of alcohol and another fluidmight be measured to determine that the proportions are present inacceptable limits, perhaps in a supply line, after mixing of alcohol orother chemical substance in a solution.

In the alternative however it might be desired to provide a filter thatprecedes the hydrogel such that unwanted chemical moieties are excludedfrom contacting the hydrogel. Thus for example certain large molecularweight moieties might be excluded by providing for a molecular weightsieve to prevent direct interaction with the hydrogel.

It is to be understood that the solution need not necessarily be asolution of a single chemical substance, although the present inventionmight be applicable to determining the concentration of a pure solution,or alternatively where an entity is unstable, it might be used to plotthe degradation of the chemical substance over time. Moreover whilstunder the above, and other, circumstances it might be desirable to beable to provide an accurate estimate of the concentration of thechemical substance, a plot of the change in concentration may be equallyuseful, rather than simply providing an absolute value of concentration.Similarly in a more complex solution any plot in the change in pressureexerted by the hydrogel is likely to be a composite of the effects oftwo or more dominant species of chemical substance, and such a changecan also be useful as a means of tracking the dynamic process such as afermentation. Such tracking might determine when further reagent is tobe added or further step is to be taken to, for example, stop thefermentation process.

Referring now to a first embodiment of the sensor (1) as shown in FIGS.1 through to 3. The sensor body (2) takes the form of a disk shapedceramic block. The sensor body includes a central bore (3) into whichare placed a hydrogel disc (5), a permeable ceramic disc (6), and aperforated stainless steel disc (7). A retaining collar (8) also ofstainless steel holds the above components in place in the bore.

The top of the sensor body includes components of a pressure transducer(4). Two piezo resistive strain gauges (9, 10) are positioned over athin flexible ceramic membrane (11) covering the bore of the sensorbody, and the remaining two strain gauges (12, 13) are positioned overthe annular portion (14) of the sensor body. Leads (for example at 15)are connected between the strain gauges and contacts (indicated as a toe) on the upper surface of the sensor body.

The sensor body and strain gauge arrangement used in this example arecommercially available as a Metallux brand piezoresistive pressuretransducer model ME 651ff with a sensitivity range from 0 to 500 Kpa.These are available from Metalux Electronics (Italy). In assembling thesensor, the hydrogel layer is inserted into the bore. The hydrogel takesthe form of a 0.3 mm disk of 2 hydroxyethyl methacrylate sold under theTrade Mark Benz as G55 by Benz Research and Development (Florida, USA).G55 is used in the manufacture of optical contact lenses.

A block of 2 hydroxyethyl methacrylate is saturated with water, held anda cutting blade slices a desired thickness. The cut piece is positionedwithin the sensor body. The permeable ceramic disk is then positioned inplace, being adhered to a thin disk of fibreglass to which is thenadhered to the sensor body. The fibreglass assist with proper adhesionto the sensor body. Following that the perforated stainless steel disk(7) is put in place. The desired pressure is then applied to theperforated ceramic disk, in the present case this was 200 kPa, whilstthe pressure is maintained a ceramic adhesive is applied, and theretaining collar is glued into place using for example and isocyanateadhesive. The pressure is applied until the adhesive sets at which timethe pressure is released. In the alternative it might be desired to drythe cut piece of hydrogel, adhere the hydrogel to the sensor body, andallow an appropriate amount of room within the sensor body forexpansion, to give the desired pressure when saturated within the body.

When placed in a 25% sugar solution with water, the pressure stabilisedat a much lower level of about 100 Kpa, and when placed in an alcoholand water solution of 25% alcohol, the pressure stabilised at about 450Kpa.

The dimensions of the sensor of the illustrated embodiment are asfollows. 18 mm outside diameter, 9 mm inside diameter with a thicknessof 6 mm. The permeable ceramic disk is kept as thin as possible toreduce lag time in measurement, and in the illustrated embodiment thepermeable ceramic disk is a 0.5 mm thick piece of aluminium oxide. Theholes within the grate are about 1.5 mm.

The general circuit for the sensor is shown in FIG. 4. The contactslabelled a to e can be seen on the diagram these are labelled in thesame manner as in FIG. 3. A voltmeter (V) can be connected betweencontacts (b) and (d) to give a read out, or the voltage signal might betaken to a computer means for storage and or display. A power source(16) in the form of a battery is connected between contact (c) and (a)bridge adjusting potentiometer (17) which balances the potentialdifference between contacts (a) and (e).

It will be appreciated that as the pressure within the hydrogel changes,the ceramic membrane will flex and there will be a differential insignal between the two strain gauges (9, 10) on the membrane whencompared with the two strain gauges (12, 13) that are positioned overthe annular portion of the sensor body. This will alter the signal sent.This change in voltage can be stored and/or displayed on a computer, andcan be used for comparisons with standards. It will be understood that arange of computational analysis of the voltage readout may be desirable.

Other forms of sensor arrangement can also be made. Strain gaugearrangements with multiple piezoresistors together with leads andcontacts are often simply printed on one side of a ceramic wafer. Oneform of the present invention contemplates also printing on the reverseside a layer of appropriate hydrogel. It is anticipated that hydrogelcompositions of this type will be available, the polymerisation of thehydrogel in this particular application may take placed in situ on theceramic wafer. A wafer so formed could then be supported in a suitablesensor body. The invention in a further aspect might be said to residein a wafer having strain gauges on one side and a hydrogel on anopposite side. The wafer might be a ceramic wafer or any other materialsuitable for use in detecting changes in pressure, by piezoresistive orother means.

Thus the thickness of the hydrogel can be quite thin. The experiment todate have been conducted on layers that have been cut to 0.3 mm drythickness. Further reduction of this thickness has to date been limitedby the machining techniques employed in the trials to date. It isanticipated that thinner layers will perform equally well, and layers ofabout 0.05 or 0.1 mm thickness might work equally well. The hydrogeldisk is fastened, and sugar or alcohol have the effect of an increase inpressure resulting across the thickness of the disk and secondly byreason of an expansion radially. The thinness of the hydrogel ispreferable because thinness leads to an enhanced reliability. Howeverthe invention could still quite readily be workable with a thickerhydrogel disk. Thus whilst the preferred embodiment contemplatesthickness of about 0.5 mm or less hydrogel disks of perhaps 3 or 5 mm ormore will also be quite adequate. Generally for reliability it ispreferred that the thickness is less than 1 mm.

The role of the permeable ceramic disk is to hold the hydrogel firmly inplace so that on a change in pressure an even pressure is exertedbetween the hydrogel and the surrounding sensor body and permeableceramic disk. Where for example the hydrogel is to be mounted directlybetween the perforated ceramic disk and the bridge of the sensor bodythen there would be localised expansion of the hydrogel disk through theperforations. That would lead to an aberrant result.

Whilst the term disk is used in respect of the hydrogel body it might bedesired to use differently shaped hydrogel bodies, and it might bedesired to make the cross sectional shape non-circular, clearly thesedifferent shapes are also contemplated by the invention. Similarly theabove description contemplates a unitary hydrogel piece, howevermultiple hydrogel particles may also be used. Thus in particular wherethe hydrogel is applied directly to a surface of strain gauge wafer asdescribed above, particles of polymerised hydrogel might be appliedwithin a permeable elastic settable material, the particles being of asize to facilitate the spraying or printed application of the hydrogel,and the settable material allowing for ready permeation of the solutionto the hydrogel but being sufficiently elastic to allow pressuretransmission resulting from the reaction of the hydrogel after contactwith the solution. Alternatively segment of the hydrogel may be piecedtogether.

The sensor body might take any appropriate form where a ceramic wafercarries multiple piezeoresistive element it will provide both a flexibleportion and a rigid portion with for suitable positioning of the piezoresistive elements. Other pressure sensing means might also be providedfor, and these might result from the use of the thicker hydrogel elementthat expands over a greater range to perceptible change. Other sensorsmight include those using piezoresistive elements in conjunction with aslicone membrane, and those that do not require piezoresistive elementssuch as copper/nickel or constantan based strain gauges.

A second embodiment of the sensor is shown in cross section in FIG. 5.The sensor (1) is intended to be screw-threaded to the end of a probethat might, for example, extend into a fermentation vat. A sensorhousing (100) is made of stainless steel and is generally in the form ofa sleeve apart from an inwardly extending flange (101) an upper portion(102) of the sensor housing is internally screw threaded for attachmentto the probe. A lower portion of the sensor is open for contact with thetest solution. The hydrogel disk is (103) is bonded to a thin ceramicdisc (104). The hydrogel is generally thicker in the centre and tapersradially and is held within a cavity in the ceramic disc. The ceramicdisc is approximately 0.5 mm and the thickest portion of the hydrogel isabout 0.2 mm thick. It is found that convenient way to shape thehydrogel disk is to form a concave hollow in one surface of the ceramicdisc, place a thin layer of saturated hydrogel over the concave hollowand apply a moderate pressure allowing the hydrogel to dry. It has beendiscovered that in doing so the hydrogel bonds to the ceramic disc. Thehydrogel can then be finished to the surface of the ceramic disc thusending up just filling the concave hollow. The hydrogel used in thisembodiment is 2-hydroxyethyl methacrylate.

A spring disc (105) is placed against a lower surface of the flange(101). The combination of the ceramic disk and hydrogel is positionedinto a recess (106) in a perforated stainless steel disc (107). Athreaded retaining collar (108) is inserted threaded into the lowerportion (109) of the sensor housing to retain the hydrogel assembly inplace. It can be seen that the retaining collar has a circumferentialflexing slot (110) formed therein to provide for some tension againstthe hydrogel assembly. A strain gauge (111) is positioned against theupstream face of flange (101). The construction of the strain gauge isvery similar to that described in the first embodiment of the invention,in that a ceramic disc (112) is positioned over an annular ceramicsupport (113). Four strain gauges are positioned in a manner similar tothat shown in FIG. 3. The strain gauge is retained in the upper portionof the sensor housing (100) by a strain gauge retaining collar (114)which is threaded into the housing and fastens the strain gauge inplace. Spring disc (105) is in direct contact with the hydrogel, and ismade of stainless steel, additionally it has a feature of a contact boss(115). The position of the contact boss is shown in FIG. 5 is where thehydrogel is in its dry state, where the boss is slightly spaced apartfrom the strain gauge. On wetting the hydrogel swells, influences thespring disc and attached contact boss to move toward the strain gaugeand make contact therewith. The swelling of the hydrogel is constrainedby the spring disc, which exerts pressure thereon. It will beappreciated that the shape of the hydrogel is shaped so as to exert anappropriate expansion of the disk, whereby a radially central portionbows further than circumferential portions.

The output of the second embodiment of the probe together with thearrangement described in FIG. 4 was then used to measure a number oftest solutions, which were made up as either weight/volume solutions orvolume/volume solutions. It can be seen that direct or indirectrelationships are found with concentration of NaCl, sucrose and alcohol(methanol). See FIGS. 6, 7 and 8 respectively. This shows the very goodcorrelation in concentration within at least certain ranges.

In these examples the alcohol may act directly on the hydrogel throughan interaction with the hydrogel structure to exert an expansion effect.The expansion effect thus is not by reason of drawing water out butrather perhaps by a replacement of water with alcohol or of furtherinteraction with alcohol in addition to water. The sugar in the otherhand exerts an osmotic effect reducing the amount of water within thehydrogel to cause a shrinkage, or reduction in pressure.

The second embodiment of the probe was also tested during wine making.It is found that over the weeks of a wine fermentation that the changein pressure output can be tracked and appears to follow estimates ofsugar content quite closely. This trial thus shows that the otherchemical substances in the complex wine making process do not adverselyaffect the capacity to track the changes that are important towinemaking. Given that trials with alcohol and sugar show that thereaction to sugar is a several times greater than that for the oppositereaction to alcohol it is not surprising that change in sugar inparticular was closely tracked as opposed to changes in alcohol.

It is anticipated that a similar finding will be made with otherfermentations where a change in sugar is tracked such as beer making, orwhere change in an alcohol is tracked such as in perhaps vinegarmanufacture.

1. A method of monitoring a wine or beer fermentation solution in afermentation vessel, comprising: holding a saturated hydrogel under abase pressure of at least 50 kPA; exposing the hydrogel to the solution;and recording changes in pressure in the hydrogel over time, wherein thehydrogel is responsive to concentrations of both sugar and alcohol andis an unsubstituted hydrogel not modified to have substituents havingparticular affinities for sugar, wherein sugar exerts an osmotic effecton the hydrogel with increasing concentrations causing a reduction inpressure, and wherein alcohol entering the hydrogel causes an increasein pressure by expanding the hydrogel.
 2. The method of claim 1 whereinthe hydrogel is an hydroxyalkyl methacrylate.
 3. The method of claim 1wherein the holding of the hydrogel includes holding the hydrogelbetween a porous rigid support and a planar compressive membercontacting a strain gauge, the hydrogel when wet exerting the basepressure against the compressive force of the planar compressive member,the solution traversing the porous rigid support to contact the hydrogelto thereby vary the force exerted via the planar compressive member onthe strain gauge.
 4. The method of claim 3 wherein the hydrogel isshaped to have a centrally thicker portion and to taper radiallyoutwardly.